The outages on Monday at Facebook, WhatsApp and Instagram likely occurred because of a problem in the company’s domain name system, an obscure but crucial component of the internet. Commonly known as DNS, it’s like a phone book for the internet. It’s the tool that converts a web domain, like Facebook.com, into the actual internet protocol, or IP, address where the site resides. Think of Facebook.com as the person one might look up in the white pages, and the IP address as the physical address they’ll find.